OpenSSL and AndroidPay

This is my first deep dive into OpenSSL and boy oh boy is it complicated and not well documented. Unfortunately that means I can’t explain a ton of the reasoning
behind the decisions I made in my code. It’s mostly like that because that’s what I found that works.

AndroidPay sends a JSON payload to a server. In that payload are 3 things the tag, the ephemeralPublicKey, and the encryptedMessage (the data). On the server side we
need the private key which is the companion to the public key that AndroidPay used to encrypt everything. With those 4 things we can then decrypt the encryptedMessage.

Step 1

We need to get the private key that resides on the server into a format that OpenSSL can use. It, along with the ephemeralPublicKey, will be used to generate a shared key
that can decrypt the message. The Java example Google provides has a private key which is created with the PKCS8 format. Luckily
OpenSSL has a handy container for that called PKCS8_PRIV_KEY_INFO there are also methods that allow us to use BIO which is OpenSSL’s IO wrapper to convert the base64 encoded key into
the PKCS8_PRIV_KEY_INFO format.

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PKCS8_PRIV_KEY_INFO*p8;BIO*bio_64=BIO_new(BIO_f_base64());BIO*bio_mem=BIO_new(BIO_s_mem());// Read merch_privkey_b64 into BIO
BIO_set_flags(bio_64,BIO_FLAGS_BASE64_NO_NL);BIO_write(bio_mem,merch_privkey_b64,(int)merch_privkey_b64_len);bio_mem=BIO_push(bio_64,bio_mem);// Read the key from the BIO
p8=d2i_PKCS8_PRIV_KEY_INFO_bio(bio_mem,NULL);BIO_free_all(bio_64);

Once we have the private key into the PKCS8_PRIV_KEY_INFO container there’s a nice method that will convert it into an EVP_PKEY which is what we want to use for our public
and private key pairs.

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EVP_PKEY*private_key=EVP_PKCS82PKEY(p8);PKCS8_PRIV_KEY_INFO_free(p8);

After we free up our p8 variable, we have our private key ready to go.

Step 2

Now we need to get the ephemeralPublicKey into that EVP_PKEY format. This key is a point on a special elliptic curve named NIST P-256 or in OpenSSL the curve is known as prime256v1.
The ephemeralPublicKey is base64 encoded so you need to decode it first (not shown). Then, with the decoded octet string we can set the point using an o2i method which
means octet to something (maybe input??) I don’t really know what the ‘i’ stands for but it does set the point correctly.

This method deals with a different type than the EVP_PKEY type we saw in step 1, so we’ll need to convert this EC_KEY type to EVP_PKEY to get our public and
private keys to play nice together. From what I understand EC_KEY is a lower level abstraction in OpenSSL and it’s advisable to only use them if you really know what you’re doing.
I don’t, so we need to convert it to the higher level EVP_PKEY. I believe EVP_PKEYs actually use EC_KEYs under the hood and there’s actually a nice method to set an EC_KEY
in an EVP_PKEY.

Now we have a public key and a private key, and both are in the same format.

Step 3

We now need to use our public and private keys to generate a shared secret key. This shared secret and the ephemeralPublicKey will get concatenated into a single buffer that we
can use to generate our decryption key. In order to create the shared secret we need to use some EVP_PKEY methods. First we’ll create a context container to hold both keys
and then derive the shared secret using that context.

As mentioned above the shared secret is concatenated with the ephemeralPublicKey. This makes up what’s called the “input keying material” for the HKDF algorithm that’s used to generate our
mac and decryption keys.

There is some slight pointer arithmetic going on to perform the concatenation. I originally tried to use strcat but these are raw byte buffers not real strings so it didn’t result in the
desired value.

After this step we should have all the things necessary to generate our AndroidPay decryption key.

Step 4

In order to generate the encryption key we need to use the HKDF algorithm with a SHA256 hash. This means we’ll need to use the SHA256 message digest (MD)
constant provided by OpenSSL. Since it’s a constant we don’t need to worry about cleaning it up at the end which is nice. We’ll then take that constant and run it through an HMAC
as per the HKDF algorithm’s extract step. According to AndroidPay
we specify a NULL salt and use our input keying material that was generated previously in the HMAC.

This gives us some pseudo-random characters that need to be put into an EVP_PKEY container. We then feed EVP_PKEY through the HKDF expand step. As per
the AndroidPay docs we use the ASCII
character string “Android” as the info component.

One thing to keep in mind is that we’re dealing with a SHA256 hash for HKDF. This results in only a single iteration through the HKDF expansion
step which greatly simplifies our code.

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unsignedcharT[EVP_MD_size(sha256)];size_tT_len;EVP_PKEY*prk=EVP_PKEY_new_mac_key(EVP_PKEY_HMAC,NULL,prc,prc_len);//pseudorandom key
EVP_MD_CTX*hmac=EVP_MD_CTX_new();// Once we've generated the pseudo random key we run
// the HKDF expand step passing "Android" in for info.
unsignedcharbyte=1;if(!EVP_DigestSignInit(hmac,NULL,sha256,NULL,prk)||!EVP_DigestSignUpdate(hmac,"Android",7)||!EVP_DigestSignUpdate(hmac,&byte,1)||!EVP_DigestSignFinal(hmac,T,&T_len)){ERROR;}EVP_MD_CTX_free(hmac);EVP_PKEY_free(prk);

After a single iteration through the HKDF expansion we should have a 256 bit filled buffer. The first 128 bit, or 16 byte, half is the symmetric encryption key, the second half
is the mac key. We’ll use the mac key to validate the tag value and make sure that we are decrypting something that actually came from
our AndroidPay client. The symmetric encryption key is used to decrypt the encryptedMessage.

Step 5

Before we do the decryption we want to make sure that the tag value that was sent with the AndroidPay payload is valid. That will indicate that this is a valid request and we are
clear to decrypt it. The tag value should be the result of an HMAC with a SHA256 hash of the previously generated mac key and the encryptedMessage. We’ll once again use the EVP_MD
for the SHA256 message digest. Make sure the encryptedMessage is base64 decoded (not shown).

Once we have the tag_comparison we then have to compare it but we need to be mindful of timing attacks here. Luckily OpenSSL provides a memcmp method that guards against timing
attacks.

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if(CRYPTO_memcmp(tag,&tag_comparison,tag_len)){INVALID;}

We’ve now validated the AndroidPay client’s data so we’re clear to decrypt.

Step 6

Decrypting the encryptedMessage is pretty straight forward if you’ve made it to this point. We just need to make sure the encryptedMessage is base64 decoded (not shown) then run it through
the appropriate cipher. According to the documentation the cipher is AES128 CTR
mode with a zero IV. I don’t know what that means but OpenSSL has functions and parameters that share that name so we can use those.

A few things we need to be mindful of, the unencrypted lengths used in EVP_DecryptUpdate and EVP_DecryptFinal_ex need to be set to something, otherwise it causes errors. Also
you can’t just reuse the same length variable for both those methods. EVP_DecryptFinal_ex isn’t additive so you need to make sure you’re moving the pointer and using a new length
variable. Finally, the base64 length we use to allocate the unencrypted_message buffer is always going to be larger than the unencrypted message data itself so we need to make sure that we
properly terminate the data at the correct spot. Otherwise you could get garbage at the end of the unencrypted message buffer.

Conclusion

I’m not entirely sure why some of these things are the way they are but once it’s all together the decryption process seems pretty logical. This was my first dive into the OpenSSL
codebase and I’m not completely convinced that I got everything correct, but, it works and hopefully doesn’t have any major issues.